1. Field of the Invention
The present invention utilizes a neural network to estimate cardiac output using thermal dilution. Specifically, an improved implementation of a standard catheter device combined with the nonlinear analyzing ability of a neural network provides unattended, semi-continuous cardiac output measurements over an extended period of time.
2. Prior Art
Hemodynamic monitoring of patients is a primary method of obtaining information for overall patient status, particularly during an operation or intensive care. Of the methods available for invasive measurement of the volume of blood pumped by the heart during a given time interval, thermal dilution remains the most common method of determining cardiac output. Popularity of the method is due in part because only a single venous catheter is required, blood samples are not needed, the thermal indicator is inexpensive, and it is a relatively fast technique.
Thermal dilution operates on the principle of conservation of energy and in determining cardiac output is referred to more specifically as thermodilution. Typically, a 5 to 10 ml bolus of cold injectate is inserted into the venous circulation of the right atrium through the lumen of a catheter. The bolus mixes with the blood and travels into the right ventricle of the heart, then passes by the thermistor located at the tip of the catheter placed in the pulmonary artery. Changes in blood temperature occur over time as the bolus is washed out of the heart, and result in an energy change in the blood flowing through the artery. Blood temperature is measured to create a thermal dilution curve from which cardiac output is derived.
The thermodilution method of measuring cardiac output is currently considered the accepted standard by physicians. However, there are many sources of error in the method, as well as problems inherent to the method itself that reduce effectiveness. For example, a typical source of error includes mishandling of injectate. Thermodilution is a function of injectate volume and temperature. Careful control of injectate administration is required to produce accurate readings, and readings can only be partially corrected for errors that do occur. Other errors are loss of indicator through the wall of the catheter, and thermal noise in the pulmonary artery. However, unlike injectate handling, correcting for thermal noise is much more difficult. These errors not only affect accuracy of cardiac output estimates, but also the repeatability of the measurement.
Despite widespread use, typical methods of thermodilution also suffer from being inconvenient and time consuming. Accurate measurements require careful set up of equipment, and close attention to injection intervals. The attending nurse or physician must also spend time waiting between injections, thus making cardiac monitoring expensive. In addition, volume loading of a patient from repeated injections of saline is hazardous when the patient suffers from congestive heart failure or renal dysfunction, thereby limiting continuous or ongoing application. The method is also not applicable for unattended patient monitoring because of the procedures involved in injecting the bolus. For these reasons, the method remains relegated to periodic spot checks, but is still used because of greater accuracy compared to noninvasive methods, and simplicity over other invasive techniques.
Several attempts have been made to improve on the method, but they too suffer significant drawbacks. For example, a heating element was introduced on a catheter that intermittently heats the blood flowing by it. However, because the amount of heat must remain low to avoid destruction of red-blood cells, the signal-to-noise ratio is high. To compensate, a number of measurements are averaged, resulting in a measurement of cardiac output that is 6 to 10 minutes old. In addition, the specialized catheter with a heat coil increases the method cost. Although the reading is semi-continuous, the infrequency of output makes this method inapplicable for many uses.
Other attempts include the injection of a room temperature bolus, or reduction of the bolus size. Although these variations on the method perhaps reduce the difficulty of handling injectate, they suffer from a low signal-to-noise ratio that makes repeatability of estimates difficult.
What is needed is a fresh approach to thermodilution estimates of cardiac output that can 1) compensate for a low signal-to-noise ratio, 2) increase applicability by reducing volume loading, 3) reduce errors caused by handling of bolus injectate or coolant as the injectate will now be referred to, 4) only require the introduction of a single catheter into the patient's heart, and 5) provide a semi-continuous output that can be used during operating procedures as well as intensive care monitoring that does not require constant supervision.